The present disclosure sets forth a method and apparatus enabling the transmission of information from a transmitter which operates without power. This particularly enables transmission from moving operative equipment which thereby transmits a variable which is encoded to transmit either analog or digital data. This involves the deployment of a transmitting apparatus in the midst of a complex mechanism of moving parts. The transmitter apparatus however is passive in the sense that it does not require a power supply, typically, batteries or the like. In many telemetry situations it is difficult, perhaps impossible, to provide electrical power to the transmitter. For instance, in rotating equipment where measurements are needed, power can be provided only by incorporating a battery with a transmitter which rotates with a transmitter. Alternately, electric power can be provided through slip rings or other communicators. There are many different techniques for doing this but they are mechanically difficult to implement and can be impossible in some situations. Certain physiological measurements also may require monitoring. For instance, medical implants can be placed beneath the skin to provide measurements of patients. This passive transmitter is able to get the signal through the skin without wires through the skin. There are a great variety of situations where this might be implemented.
The present apparatus is a passive or unpowered transmitter system. It features a passive transmitter which is defined by one or more elements which exhibit variable resonant frequencies. As an example, each element may be a tank circuit which is made up of a connected inductor and capacitor. For the purpose of illustration, consider tank circuit elements having relatively high Q values of perhaps 50 or higher. This is superficially similar to, but, markedly different from the passive tag system used in retail stores for theft protection. There, expensive garments and the like are equipped with a passive tag which is an energy absorbing system tuned to a particular and known frequency. If the clothing is stolen with the tag still attached, the tag interacts with the transmitters at the portals of the store so that the gateway coils connect with the tag, energy is absorbed at the tag, and this sounds an alarm. In this situation, the resonant frequency of the circuit involved with a tag is known. The tags are fixed at their resonant frequency and the detection system recognizes only those frequencies.
The present apparatus sets forth a system where the frequency is not known. The resonant frequency of the passive transmitter exemplary tank circuit is indeed the variable to enable encoding of variable information in the form of an analog or digital signal. Consider as an example a system which provides an output of discrete measurements. For the purposes of example, assume that the variable to be measured is weight of a particular material in a mixing device. The mixing device is placed on a set of scales which operates by means of suitable mechanical connectors. The scales convert the weight into a value of capacitance. If the scales provide measurements of ten different increments, for instance, the weight can vary between 200-300 pounds, the scale might provide an indication of weights of 200, 210, 220, 230, etc. In this instance, the tank circuit readily using an adjustable capacitor, the capacitor providing values of capacitance given by the relationship of A+nB where A is a fixed capacitance corresponding to 200 pounds, B is an increment capacitance corresponding to 10 pounds and the value of n is a whole number integer between 0 and 9. In the foregoing example where the capacitance is installed parallel with a suitable inductor, this will vary the resonant frequency of the tank to any of several values which correspond to the capacitance. The frequency values will be denoted generally as f.sub.0, f.sub.1, f.sub.2 . . . f.sub.n. These values obviously depend on the incremental change in capacitance so that the spacing might be a few kilohertz between adjacent frequencies. In the foregoing example, one does not know a priori the value of the tank capacitance and hence the resonant frequency of the tank circuit. However, the range is known in this particular example and is defined by the limiting frequencies F.sub.1 and F.sub.2 which encompass or include all of the frequencies f.sub.0, f.sub.1, f.sub.2 . . . f.sub.n representative of resonant frequencies of the tank circuit from the minimum to the maximum weight. Again this enables a signal to be obtained indicative of the variable transmitted from the passive transmitter.
The present system utilizes a timed based sweep between the frequencies F.sub.1 and F.sub.2. This will repetitively interrogate the tank circuit. At some unknown frequency, the tank circuit will achieve resonance. When it does, it absorbs energy from the receiver apparatus of the system thereby identifying resonant energy. Assuming that a relatively high Q tank circuit is used, the loading is fairly well defined. If, for instance, the Q is 100 or greater, a relatively sharply defined frequency is then identified. This can be quite easily discriminated in the receiving apparatus so that the frequency of the tank circuit is then known. Suitable scale factors enable conversion in this particular example from resonant frequency to weight on the scales. As will be understood, there is a field coupling involved which thereby enables the transmission to occur from the passive transmitter to a receiver system of data which can be calibrated as described.
Considering the present application further, it is particularly useful where the variable changes periodically. Assume that it is desirable to provide a sample every 10 seconds or 6 samples per minute. The present apparatus can be operated to scan or sweep between the frequencies F.sub.1 and F.sub.2 periodically, e.g., every 10 seconds. That is, the scan can be controlled in speed to require the exemplary 10 seconds where the scanning cycle is repeated time and time again. This provides a data which is interrogated at the desired rate for easy use.